As is generally known, telecommunications systems employ a cellular system in which a plurality of radio basestations each maintain one or more “cells” to which user equipments (also known as mobile terminals) are connected. The radio basestations send communications (e.g. control and data signals) to the UEs in the downlink, and receive communications from the UEs in the uplink. The radio basestations further communicate with a core network, which maintains overall control of the telecommunications system. In different telecommunications systems, functionality is split differently between the radio network (i.e. the radio basestations and the mobile terminals, etc) and the core network.
FIG. 1 shows an exemplary telecommunications system 10, known as the E-UTRAN (Evolved UMTS terrestrial radio access network) which uses the Long Term Evolution (LTE) standard. The system 10 comprises a plurality of radio basestations (also known as eNode Bs, Node Bs, etc) 12a, 12b, 12c, each of which maintains one or more cells (not illustrated). UEs 14a, 14b, 14c, 14d within each cell communicate with the corresponding radio basestation 12 of that cell. Also, as is known, cells are grouped together into what are known as “tracking areas”.
In the E-UTRAN, radio basestations are capable of communicating with one another over interfaces known as X2 interfaces (illustrated as dashed lines in FIG. 1). Each radio basestation 12 further has one or more interfaces with the core network. These are known as S1 interfaces (illustrated as solid lines in FIG. 1). In particular, the radio basestations 12 have one or more S1 interfaces to one or more mobility management entities (MMES) 16a, 16b, as will be described in more detail below.
The cells of a radio basestation can be connected to different sets of MMEs (i.e. different MME Pools), through use of the tracking area. One MME pool is responsible for a certain tracking area, and a cell is connected to a certain tracking area. In FIG. 1, all of the cells of the radio basestations are connected to the same MME Pool (i.e. MMEs 16a and 16b). However, different cells, of the same radio basestation, may be connected to different tracking areas that are in turn connected to different MME pools.
A UE generally selects its cell by determining the strongest received signal and, once selected, the UE reads system information through that cell. The system information contains, amongst other things, the tracking area of which the cell is part.
Once the UE selects a cell, it also performs a tracking area update towards the core network (e.g. the MME) to let the core network know in which tracking area it is located, and to receive a tracking area list of the cells in which, should the UE move to any of them, the UE need not perform a tracking area update.
Thus, it can be seen that a relatively high amount of traffic may be generated to and from the core network when a UE moves between cells.
A further problem arises when all of the S1 interfaces of a particular radio basestation have become non-operational. A proposed solution to this problem has been to “lock” the cell, i.e. to effectively turn off the output power of the radio basestation, and thus to stop broadcasting the system information. However, this solution itself causes problems. For example, a user in idle mode that would not be able to re-select to another, functional cell will become aware that the network has developed a fault, in that his or her UE will indicate that there is no network. Furthermore, if an S1 interface becomes non-operational, it will most probably be affecting many radio basestations at the same time. This implies that many UEs will attempt cell-reselection, with some or all of them performing a subsequent tracking area update, at the same time. This could potentially lead to failure in other nodes that become inundated with connection requests, etc, causing the problem to spread to other parts of the network.